Most superconducting microwave devices continue to be fabricated from epitaxial thin structures deposited on single crystal substrates. The need for better lattice-matched, cubic, low dielectric constant and low microwave loss substrates and buffers continues. Heretofore, a popular substrate or barrier dielectric in thin film superconductor technology has been Lanthanum Aluminate, known as LaAlO.sub.3, but it is being challenged by Lanthanum Strontium Aluminum Tantalate, known as LSAT, which is a solid solution of 30 mole % LaAlO.sub.3 and 70 mole % Sr.sub.2 AlTaO.sub.6. LSAT overcomes several of the drawbacks associated with LaAlO.sub.3, because it is cubic, does not undergo phase transition and has a slightly lower dielectric constant. Also, LaAlO.sub.3 places High Critical Temperature Superconductor (HTSC) films in compression, an advantage for brittle films having a poor thermal expansion match to the substrate. The LSAT lattice parameter falls between that of the a and b parameters of YBCO and introduces less stress than LaAlO.sub.3, providing an even greater advantage for use in HTSC films. The dielectric constant for LaAlO.sub.3 is 24, while for LSAT it is 22 and both are relatively large. Thus an optimum substrate or buffer would be one having a much lower dielectric constant such as those listed in TABLE I, which range from 9.5 to 15.0.
Prior investigations by the inventors herein and others have been made with ordered perovskites of the A.sub.2 MeSbO.sub.6 compounds where A is barium or strontium and Me is a trivalent ion that is often ordered on octahedral sites of those compounds. These findings are included in patent applications CECOM Docket No. 5304, entitled "Rare Earth Metal Compounds For Use In High Critical Temperature Thin Film Superconductors, Ferroelectrics, Pyroelectrics, Piezoelectrics, and Hybrids," U.S. patent Ser. No. 08/717,822 and CECOM Docket No. 5151, entitled "Compounds In the Series A.sub.2 MeSbO.sub.6 For Use As Substrates, Barrier-Dielectric Layers And Pasivating Layers in High Critical Temperature Superconducting Devices," U.S. patent Ser. No. 08/502,739, both of which are incorporated herein by reference. Because pentavalent ions have a small polarizability, the dielectric constant for these compounds are much smaller than those observed in either LaAlO.sub.3 or LSAT.
It is well-known that the polarizability of Ca 2+on A sites in perovskite is 30% smaller than Sr.sup.2+ and 100% smaller than Ba.sup.2+. Since Ca.sup.2+ is smaller than Ba.sup.2+ or Sr.sup.2+, its size affords a better opportunity to control the lattice parameter. Due to these properties of Ca.sup.2+, it is theorized that compounds in the system Ca.sub.2 MeSbO.sub.6, where Me is a 3+ ion selected from the group consisting of aluminum (Al), scandium (Sc), indium (In), gallium (Ga), or a rare earth metal would provide a lattice matched, cubic perovskite with a low dielectric constant ranging between 9.5 and 15.0 (see TABLE I) that would overcome the long-felt shortcomings, drawbacks and limitations observed in both LaAlO.sub.3 and LSAT. The Al, Sc, In, Cr, Mn and rare earth containing compounds were first prepared by Fesenko, et. al. and they found that the Al, Sc, Cr, Fe and Mn containing compounds were cubic and the In compound was monoclinic, with the lattice parameters given in Table I, and they also found the rare-earth compounds to be monoclinic. The crystallographic observations made by the present inventors are in some cases different than the literature. In addition, the present inventors have determined that the properties of some of these compounds in the system Ca.sub.2 MeSbO.sub.6 satisfy the requirements for use as substrate and buffer/dielectric layers in accordance with the present invention, for growth of epitaxial YBCO films, multilayer and device structures, without suffering from any of the disadvantages, drawbacks, limitations and shortcomings of other materials like LaAlO.sub.3 and LSAT.
Prior art in this area is found at A. W. Sleight and R. Ward, Inorganic Chemistry, 3 rd edition, p. 292 (1964);
E. G. Fesenko, et. al., "Synthesis and Study of A.sub.2 Sb.sup.5+ BO.sub.6 and A.sub.3 Sb.sup.5+ BO.sub.6 Type Ternary Oxides with Perovskite Structure," Izvestia Akademii, Nauk SSSR, Neorganicheskie Materialy, 6(4), 800-2 (1970), Rostov, Gos. University, Rostov, USSR; PA0 R. D. Shannon, et. al., "Dielectric Constants of Yttrium and Rare Earth Garnets, the Polarizability of Gallium Oxide, and the Oxide Addivity Rule," 67(8) Journal of Applied Physics, 3798 (1990); PA0 R. D. Shannon, et. al., "Dielectric Polarizabilities of Ions in Oxides and Fluorides," 73(1) Journal of Applied Physics, 348 (1993); PA0 V. J. Fratello et. al., "Calculation of Dielectric Polarizabilities of Perovskite Substrate Materials For High-Temperature Superconductors," 9(10) Journal of Materials Research, 2554 (1994); and PA0 S. C. Tidrow et. al., "Dielectric Properties of Perovskite Antimonates," 7(2) IEEE Transactions on Applied Superconductivity, 1769 (1997).